We previously identified in fetal rat serum a binding species capable of specifically binding 125I-IGF-II that is considerably larger than the 150- and 40-kDa carrier proteins. We now have immunologic and affinity crosslinking data to show that this binding species is the type II IGF receptor. Rat serum was gel filtered on a Sephadex G-200 column (0.05 M NH4HCO3, pH 8), and 125I-IGF-II binding was measured in individual column fractions. 125I-IGF-II binding activity was found in the void volume (Vo) in addition to the carrier protein regions. Competitive binding studies using 125I-IGF-II and bonding activity from the G-200 vo showed the characteristics of the type II receptor: IGF-II was more potent than IGF-I, and insulin did not compete. Importantly, a specific anti-type II receptor IgG that recognizes neither the 40- and 150-kDa serum IGF carrier proteins nor the type I IGF receptor completely blocked 125-I- IGF-II binding. 125I-IGF-I did not bind to the Vo fractions, demonstrating absence of the type I IGF receptor. Independent support for identification as the type II IGF receptor tor came from affinity crosslinking experiments using disuccinimidylsuberate. Crosslinking of 125I-IGF-II to the G-200 Vo material demonstrated a specific band at 210 kDa without reduction and 240 kDa with reduction of disulfide bonds. The sizes was confirmed by Western blotting of G-200 Vo material with the anti-type II receptor IgG which revealed a band slightly smaller (10 kDa) than the type II receptor from rat placental membranes. Immunoquantitation by Western blotting using pure type II receptor from rat placental membranes as standard showed a development pattern. In fetal rat serum (19 days gestation) and in sera from 3- and 10-day-old rats, 1-5 g/ml receptor protein was measured. The levels of type II receptor declined dramatically between age 20 days and 40 days, but receptor was still measurable at age 12 months. We conclude that the type II IGF receptor' tissue source and role are not defined; it is possible that the circulating receptor modulates IGF action.